The present invention relates to a process and apparatus for treating the inner surface of a vacuum chamber, and more particularly to a process for treating the inner surface of a vacuum chamber for use as a vacuum vessel such as a vacuum chamber in a charged particle accelerator to reduce gas desorption from the inner surface of the vacuum chamber produced by at least either thermal desorption or photodesorption.
In order to accelerate charged particles with high energy in a charged particle accelerator, it is necessary to prevent charged particles from scattering because of a collision of charged particles with residual gases, that is, to prevent the loss of such charged particles. The environment in which the charged particles are accelerated should be set in a ultrahigh vacuum to prevent such a loss. Consequently, a vacuum vessel such as a ultra--clean vacuum chamber and the like is employed in the charged particle accelerator to actually attain a high or ultrahigh vacuum environment. Moreover, the most important task is to reduce the gas desorption from the vacuum vessel itself in addition to increasing the pumping speed of a vacuum pump to achieve a high or ultrahigh vacuum.
In this case, aluminum alloy, stainless steel, copper or the like is normally used for the vacuum vessel, called a vacuum chamber, of the charged particle accelerator. A factor governing gas desorption from the vacuum vessel using such a metal is thermal desorption in which absorbed molecules on the inner surface of the vacuum chamber are caused to be desorbed by thermal energy.
However, what greatly affects the pressure in the charged particle accelerator other than the normal thermal desorption originates from the presence of high-energy particles in the vacuum vessel. For example, electrons and positrons whose orbits have been altered by a bending magnet or the like in a electron storage ring generate electromagnetic waves called synchrotron radiation due to radiation. The inner wall of the vacuum chamber is irradiated with the synchrotron radiation, which causes gas desorption called photodesorption from the inner surface of the vacuum chamber.
With respect to photodesorption, a description has been given in "Vacuum" (volume 33, number 7 (1983) pp. 397-406). The gas desorption caused by the photodesorption raises the pressure in the vacuum chamber and results in introducing the scattering and attenuation of stored electrons.
In order to reduce gas desorption as much as possible, there have been proposed various methods of treating the inner surfaces of vacuum chambers for the purpose of not only cleaning the inner surfaces of vacuum chambers but also removing chemical compounds and contaminants causing such gas desorption. Chemical treatments using acid cleaning, alkali etching and the like are most common by used methods of treating the inner surfaces of vacuum chambers. Chemical treatments of the sort mentioned above have also been referred to in "Vacuum" (volume 38, number 8-10 (1988) pp. 933-936).
In addition to chemical treatments, there is a discharge cleaning method in which the inner surface of a vacuum vessel is bombarded with the ions generated by electric discharge. Moreover, another one known as a pre-baking treatment method comprises the steps of heating a vacuum chamber at high temperatures in a vacuum furnace to remove compounds on the inner surface of the vacuum chamber by evaporating them, and removing the gas contained in the material of the vacuum chamber by diffusing the gas so as to discharge the gas from the material. In this way, attempts have been made to reduce gas desorption from the inner surface of the vacuum chamber by cleaning the inner surface thereof.
In the aforementioned prior art, a lubricant may be used to reduce the friction between the material of a vacuum chamber and a plug or the like when a billet is formed by extrusion into a vacuum vessel such as the vacuum chamber of a charged particle accelerator. In this case, it is feared that a contaminated layer is formed on the inner surface of the vacuum chamber because of the lubricant. Thermal desorption or photodesorption may thus cause gas desorption from the contaminated layer.
When a vacuum chamber is formed by hot extrusion, moreover, a contaminated layer as a source of gas desorption may be formed as air and impurities react on the inner surface of the high-temperature vacuum chamber. In a case where a vacuum vessel is manufactured by roll-forming out of a rolled sheet, moreover, there also arises problems that such a contaminated layer is formed during the step of producing the sheet material by rolling.
In order to reduce gas desorption from the contaminated layer due to thermal desorption or photodesorption, the process of chemically treating the inner surface of the vacuum chamber needs changing depending on the material used when the inner surface is subjected to the chemical treatment. Nevertheless, there still arise problems that since chemicals are used, the inner surface of the vacuum chamber tends to become coarse in exchange for removal of the gas desorption layer, that the scale of facilities necessary for the inner surface treatment tends to become larger since those for rinsing the chemicals used for processing purposes and preventing environmental pollution are required, and that a compound layer as a source of gas desorption is newly formed on the inner surface.
In the case of the discharge cleaning utilizing ion bombardment, on the other hand, the gas itself used for discharge is allowed to penetrate into the vacuum chamber material and the problem is that the material itself has to be removed by sputtering. Further, the high-temperature heat treatment called pre-baking may incur a reduction in material strength since the vacuum chamber material passed through the heat treatment in the vacuum furnace becomes softened. Particularly, aluminum alloy, for example, is not fit for the pre-baking treatment in view of material strength as it may melt down. In addition, no consideration has been given to energy saving notwithstanding the use of such an energy-consuming vacuum furnace.
Incidentally, the vacuum chamber as an object for patent herein is generally as long as several meters and hardly fitted in a processing apparatus after it undergoes so-called machining such as boring. In other words, it has been difficult to treat the inner surface of such a vacuum chamber.